A scanning electron microscope (SEM) and a scanning transmission electron microscope (STEM) are combined with an electron optical device such as an electron lens to form an extremely small electron beam crossover (hereinafter, will be referred to as a beam probe) on the plane of an observed sample. Transmission scattered electrons, reflected electrons, secondary electrons, or derived X-rays from a small region irradiated by the beam probe are measured to obtain information on the structure and composition of the small region. Furthermore, the electron microscope two-dimensionally scans the beam probe on the plane of a sample by means of an electromagnetic electron beam deflector, obtaining a two-dimensional image (a so-called electron microscope image).
In recent electron microscopes, an advanced technique of aberration correction allows the provision of aberration correctors for compensating the aberrations of an objective lens, e.g., a spherical aberration and a chromatic aberration. Thus, an extremely small beam probe can accurately form an image on the plane of the sample (e.g., see Patent Literatures 1 and 2).
Specifically, the resolutions of electron microscopes have improved. For example, in recent years, a commercially available STEM device having a spherical aberration corrector can obtain a resolution of 0.1 nm or less that is smaller than a typical atomic size by the aberration correcting effect of the aberration corrector. Conversely, there has been an increasing demand for a measuring method for accurately evaluating the aberration state of an electron microscope including an aberration corrector. Thus, some accurate aberration measuring methods have been developed and utilized in parallel with the development of aberration correctors.
Conventionally known aberration measuring methods include an aberration measurement using displacement due to aberrations and a probe tableau method. Another aberration measuring method is an aberration measuring method using a Ronchigram (Ronchigram method).
The aberration measurement using displacement due to aberrations and the probe tableau method require repeated measurements for reducing errors. Thus, it is necessary to expend much time and effort to obtain a set of aberration coefficients.
In the aberration measuring method using a Ronchigram, an aberration can be fundamentally measured from a Ronchigram obtained by a measurement. For example, Patent Literature 3 describes, as an example of the aberration measuring method using a Ronchigram, a method of obtaining a Ronchigram using an amorphous thin-film sample, and then obtaining an autocorrelation in the local region of the Ronchigram, achieving a local strain tensor of the Ronchigram.
Ronchigram is a projected image and thus a measured image can be obtained in a shorter time than a scanned image in the foregoing two methods.
Therefore, an aberration can be measured at a high speed by using a Ronchigram. This method is quite promising in the adjustment of an aberration corrector requiring repeated aberration measurements.